Microelectromechanical (MEM) devices are microminiature devices formed on a substrate using fabrication process steps common to the integrated circuit (IC) fabrication industry. These MEM devices generally combine electrical and mechanical functionality to form devices such as accelerometers, sensors, motors, switches, coded locks, flow-control devices, etc. In some MEM devices, chemical functionality can also be provided to form chemical sensors.
The fabrication of MEM devices is generally based on the deposition and photolithographic patterning of alternate layers of polycrystalline silicon (also termed polysilicon) and a sacrificial material such as silicon dioxide (SiO.sub.2) or a silicate glass (e.g. deposited from the decomposition of tetraethylortho silicate, also termed TEOS). Vias can be etched through the sacrificial material to provide anchor points to the substrate and between adjacent polysilicon layers. The polysilicon layers can be patterned and built up layer by layer to form various members of the MEM device structure. Once the MEM device structure is completed, the sacrificial material is partially or completely removed (e.g. by exposure to a selective etchant which does not attack the polysilicon layers) to release the polysilicon members of the MEM device for operation.
For proper functioning and reliability of the MEM device, it is desirable that the polysilicon members do not deform after release due to residual stress in the members. Such unwanted deformation can be a source of wear (e.g. between a moveable member and one or more adjacent fixed or moveable members), fatigue or cracking that can adversely affect the reliability of the MEM device.
In an attempt to reduce the residual stress a deposited polysilicon layer used to form particular members of a MEM device, an annealing step can be used after deposition of that layer. Similarly, an annealing step can be provided after fabrication of the MEM structure and before the etch release step. However, these annealing steps do not adequately reduce the stress in the polysilicon members, especially for MEM devices comprising several layers of structural polysilicon and having polysilicon members for which minimal stress is essential to proper functioning. The reason for this is that the sacrificial material itself is generally stressed to a substantial degree (e.g. a thin-film layer of silicon dioxide can have about 0.3-0.4 gigaPascals of residual compressive stress after deposition and annealing) and can impart stress to the polysilicon layer or members formed therefrom during repeated thermal cycling (i.e. during deposition and annealing steps).
The level of stress inherent in silicon dioxide or silicate glass sacrificial layers can also limit the number of structural polysilicon layers that can be used to build up a MEM device since an accumulation of compressive stress in multiple sacrificial layers can lead to wafer deformation (i.e. a distortion or bowing of the substrate). Such wafer deformation is problematic since it can result in fracture of the wafer, or in handling problems due to nonplanarity of the wafer. Additionally further photolithographic processing steps can be degraded as a result of the wafer deformation which presents a nonplanar surface of the wafer that can affect reticle image transfer and degrade feature resolution in the MEM device. Wafer deformation can also detrimentally affect the focusing capability of an autofocusing photolithographic stepper used to provide the reticle image transfer.
The practical effects of stress within conventional sacrificial materials used heretofore is thus to reduce the production yield of MEM devices, and to further to limit the number of structural polysilicon layers that can be used to form particular MEM devices. At present, MEM devices based on the use of conventional sacrificial materials are typically limited to only a few (three or four) structural layers of polysilicon, with each layer being about 1-2 .mu.m thick. This limits the complexity and functionality of the MEM devices which can be built since more complicated devices require more structural layers of polysilicon from which to form interconnected moveable members such as linear or rotary motors, linkages, actuators, gear trains, moveable mirrors, etc. In our experience, the fabrication of a MEM device having four structural layers of polysilicon using a conventional sacrificial material can only be accomplished by deviating from a standard processing sequence used for the first three structural polysilicon layers due a substantial level of accumulated stress in the MEM structure which is not relieved by an annealing step.
What is needed is a way of removing the impediment presented by compressively-strained conventional sacrificial materials, including silicon dioxide and TEOS, to the fabrication of present and future MEM devices, thereby allowing the fabrication of MEM devices of increased complexity and having a greater number of structural polysilicon layers.
An advantage of the method of the present invention is that a residual stress buildup in a sacrificial material can be overcome to allow the fabrication of MEM devices having reduced stress in one or more polysilicon members thereof.
Another advantage of the present invention is that the silicon oxynitride sacrificial material can have a composition which is thermally stable so that little, if any, stress will be produced within the sacrificial material as a result of a large temperature change produced during a thermal annealing process step.
A further advantage of the present invention is that the use of silicon oxynitride can provide a sacrificial material for which the composition can be adjusted as needed to provide a predetermined level of stress in each sacrificial layer.
Still another advantage of the present invention is that silicon oxynitride can be used as a sacrificial material throughout a MEM device, or only in selected locations, thereby limiting the accumulation of stress which can otherwise lead to wafer deformation which is detrimental to the fabrication process.
Yet another advantage of the present invention is that the use of silicon oxynitride as a sacrificial material can reduce residual stress in one or more polysilicon members of a MEM device, thereby reducing structural deformation of the members upon release and substantially improving reliability of the MEM device.
These and other advantages of the method of the present invention will become evident to those skilled in the art.